The invention relates to thermoplastic molding compositions comprising    A) from 10 to 99.94% by weight of a polyimide,    B) from 0.05 to 5% by weight of a polyethyleneimine homo- or copolymer,    C) from 0.01 to 20% by weight of iron powder, and    D) from 0 to 70% by weight of further additives,where the total of the percentages by weight of A) to D) is 100%.
The invention further relates to the use of the molding compositions of the invention for the production of fibers, or films, and of moldings of any type, and also to the moldings thus obtainable.
Thermoplastic polyamides, such as PA6 and PA66, are often used in the form of glassfiber-reinforced molding compositions as structural materials for components which during their lifetime have exposure to elevated temperatures, and this results in thermo-oxidative degradation phenomena. Addition of known heat stabilizers can delay the occurrence of the thermo-oxidative degradation but cannot prevent it in the long term, and an example of this is seen in a fall-off of mechanical properties. It is highly desirable to improve the resistance of polyamides to heat-aging (HAR), since this can give longer lifetimes for components subject to thermal stress or can reduce their risk of failure. As an alternative, improved HAR can also permit the use of the components at higher temperatures.
Kunststoff Handbuch [Plastics Handbook] 3. Technische Thermoplaste [Engineering thermoplastics], 4. Polyamide [Polyamides], 1998 Carl Hanser Verlag Munich, Vienna, edited by L. Bottenbruch, R. Binsack discloses the use of various heat stabilizers for polyamides. The use of hyperbranched polyethyleneimines in thermoplastic polymers is disclosed by way of example in DE 10030553 and DE 102005005847.
EP 1065236 discloses unreinforced polyamides in which polyethyleneimines and oligocarboxylic acids are used during the polymerization reaction. The molding compositions described have improved solvent resistance, but HAR is not fully satisfactory.
The use of elemental iron powder in polyamides is disclosed in WO 2006/074912 and WO 2005/007727.
Resistance to heat-aging remains inadequate in the known molding compositions, in particular over prolonged periods of thermal stress.